It has recently been clarified that a nano polycrystalline diamond sintered object has hardness exceeding natural single-crystal diamond and has a property excellent as a tool. Though the nano polycrystalline diamond is essentially an insulator, a further function such as conductivity can be provided to diamond by adding other elements such as an appropriate dopant. In addition, by appropriately selecting an element to be added to diamond, various characteristics of diamond such as optical characteristics, electrical characteristics, and mechanical characteristics can be varied.
For example, a method of adding a dopant to graphite by forming a solid solution thereof is available as a method of adding a dopant capable of providing conductivity to diamond, as shown in E. A. Ekimov et al., Nature, Vol. 428 (2004), 542 to 545.
Though the nano polycrystalline diamond is essentially an insulator as described above, conductivity can be provided to diamond by adding an element serving as an acceptor to diamond. The document above describes a method of synthesizing diamond to which boron has been added.
In addition, conductivity can be provided to diamond by adding an element serving as a donor to diamond. For example, Japanese Patent Laying-Open No. 2010-222165 describes a diamond layer containing an element capable of providing conductivity. Though electrons can be emitted from n-type diamond in particular, it has been impossible to obtain n-type diamond doped with a donor at a high concentration through high-temperature and high-pressure synthesis. In order to solve this problem, an example where doping with phosphorus is carried out with vapor phase synthesis (CVD) has been reported. With this method, however, it is extremely difficult to achieve doping at a high concentration or to introduce a dopant other than phosphorus.